The metallization of dielectric substrates is well established in the art, and is performed by methods such as electrodeposition, electroless deposition, vacuum metallizing and cathode sputtering. In a substractive process using these methods in the manufacture of printed circuit boards, in a first step the entire surface of the substrate board is metallized, whereupon a photoresist is applied and patterned, exposing the metal surface from which the metal is to be removed, after which the exposed surface metal is etched away. In contrast to such substractive methods, additive printed circuit manufacture provides for deposition of the metal at the pattern areas only, thus avoiding waste of metal material.
The most versatile metallization methods can be used both for substractive and additive printed circuit formation. One such method comprises the use of conductive pastes containing conductive metal powder, binder and liquid organic vehicle for adjusting viscosity. Such pastes are applied to the substrate in a desired pattern and fixed by curing the binder. The presence of binder in the cured metallic surface layer reduces the conductivity. Other methods have been proposed omitting the use of a binder, wherein a metal powder is evenly spread on the substrate surface, optionally using a liquid volatile vehicle, and subsequently compressed into the substrate surface layer. Since the substrate is required to be deformable, such methods cannot be used for all types of substrates. Moreover, commercially available powders of the cheaper and economically more attractive metals, such as copper and nickel, usually are covered by an oxide layer and, therefore, cannot directly be used. Thus, according to U.S. Pat. No. 4,614,837, to Kane et al., issued Sep. 30, 1986, prior treatment of the oxide-coated metal powders with a hydrogen reducing agent is required to accomplish the metal powder compression method described above. EP-A-297,677 of Parr et al., published Jan. 4, 1989, describes a method of forming a conductive metal layer on a substrate by depositing a layer of copper or nickel particles on a temporarily deformable substrate, contacting these metal particles with a specific kind of developing agent, and then subjecting the metal particles and developing agent to heat, in the substantial absence of oxygen, for a time duration sufficient to improve the conductivity of the metal layer. The metal particles as initially applied in Parr et al. can have an oxide layer on them without the requirement of treatment with hydrogen prior to use.
EP-A-280,918 of Mehherjee et al., published Sep. 7, 1988, discloses in-situ decomposition of organometallic compounds at the surface to be metallized. However, this method uses a binder, and the precipitated metal itself does not provide a conductive layer, but rather serves as catalyst for further traditional plating steps.
According to JP-A-72/21569 (Derwent abstract 40748t) an alumina substrate is precoated with a copper sulfide kaolin composition, whereupon a silver carbonate layer is applied, which is reduced by heating at temperatures above 700.degree. C. in a reducing atmosphere. The high temperature requirement precludes the use of the method for resin substrates. SU-A-362,070 (Derwent abstract 49584u) discloses the metallization of dielectrics by chemical deposition of copper metal using an aqueous solution of metal salts and reducing agents.
JP-A-88/125,605 (Derwent abstract 88-187,417) discloses the production of fine metal powders by gradually mixing and decomposing a non-aqueous solution of a nitrate, sulfate or chloride of certain metals with a non-aqueous solution or dispersion of a reducing agent, such as hydrazine or a mixture of hydrazine with at least one of boron hydride, formalin, sodium pyrophosphite and triethanolamine.
Copending U.S. patent application Ser. No. 447,779, filed Dec. 8, 1989, discloses a method for effecting reductive deposition of a metallic coating onto a substrate by heating a mixture of a metal salt or metal compound and amine compound having at least one functional substituent group capable of coordination to and reduction of metal ions, the amine nitrogen and the functional substituent being separated by from two to six other atoms. Although the method of this reference produces excellent results, it does have limitations such as with respect to speed, potential for automation, and production of highly resolved patterns.
U.S. Pat. No. 4,790,912 (Holtzman et al.) discloses selectively depositing metals on a non-conductive substrate by charging a catalyst on the substrate with hydrogen electrolytically generated from a protic bath and contacting the catalyst/hydrogen combination with a solution of metal salt. There are amine adjuvants (promoters) that may be used in the electrical plating bath of Holtzman et al., but they do not effect reductive depositions.
There is also considerable prior art that discloses the use of photochemical energy as provided by a diffuse light source or coherent, high energy laser beam to effect deposition of metals on a substrate.
U.S. Pat. No. 4,526,807 to Auerbach, issued Jul. 2, 1985, discloses a method of depositing conductive elemental or metalloid spots on a non-conductive substrate. At least one reducible metal or metalloid compound is placed in a solution or dispersion comprising an oxidizable organic matrix. The organic matrix typically comprises a polyamic acid or a polyimide. The substrate is coated with the solution or dispersion, and the coated substrate is then contacted with a beam of localized radiation absorbable by the coated substrate, thereby causing reduction of the metal or metalloid to the elemental state. The organic matrix is described generally as comprising at least one nitrogen compound, typically containing amine, amide or similar functionalities. When the metal to be deposited is subject to oxidation under ambient conditions, e.g. copper, Auerbach recommends an inert or reducing atmosphere.
U.S. Pat. No. 3,451,813 to Kinney et al. describes the use of high intensity photoflashes to cause formation of conductive deposits from a variety of metal salts mixed with a radiation sensitive pyrolizable organic resin. There is no mention of amines.
U.S. Pat. No. 4,636,403 to Fisanick et al. describes the use of laser energy to repair metallic and non-metallic photomasks via deposition of metal from a metal-organic compound. This reference does not suggest use of metal salt-amine mixtures, but employs metal-organic compounds such as a gold terpene mercaptide. Because of the nature of the metal-organic compounds used, the laser output power must be ramped up rather than pulsed.
U.S. Pat. No. 4,578,155 to Halliwell et al. describes electroless plating by submerging the substrate in the plating solution and passing a laser beam through the solution to cause a metal deposit to be made on the substrate. The substrate must also remain immersed in the solution during the deposition process.